1. Technical Field
The disclosure relates to a lubricating oil composition and method for manufacturing the same, and in particular relates to a lubricating oil composition in the absence of a dispersant (or surfactant) and method for manufacturing the same.
2. Description of the Related Art
Environmental pollution is one of the most-discussed issues in the world today. In particular, CO2 content in the atmosphere is considered one of the causes for global warming. An improvement in operating efficiency of a machine can directly lead to energy savings and indirectly reduce the emission of carbon dioxide. The improvement of operating efficiency a machine can be achieved by effectively lowering the frictional losses in mechanical devices by providing a low coefficient of friction, and lubricating oils have been found to be useful in reducing friction in mechanical devices. The improvement in efficiency depends primarily on the friction properties of the lubricating oil. Further, lubricating additives can be incorporated into the lubricating oil for further improving the friction properties thereof.
The advanced commercially available lubricating additives include organic compound contained sulfur, phosphorous and chloride, such as molybdenum dithiocarbamate (MoDTC) or molybdenum dithiophosphate (MoDTP) as disclosed in Japanese Patent Provisional Publication No. 8-20786. However, those additives have disadvantages of low thermal and pressure resistances, and short lifetime and cause an environmental contamination problem.
To the contrary, inorganic solid additives, such as graphite, molybdenum disulfide, or nanodiamond, exhibit high thermal and pressure resistances and durability. However, since molybdenum disulfide easily be oxidized and causes an environmental contamination problem, the use of molybdenum disulfide has been prohibited by law in recent years. Further, due to the large size, graphite additives are apt to cause precipitation and consequently result in blocking.
Ultra dispersed diamond (UDD) is a structural combination of a diamond core and graphite layer surface and can be fabricated by detonation method. The ultra disperse diamonds have particle sizes between 4 to 6 nm, and surfaces of the ultra disperse nano-diamonds are covered by a fullerene-like carbon, which aggregates into particles of hundreds of nanometers in diameter. The ultra dispersed diamonds are not only hard, they also have extremely high thermal conductivity, high wear-resistance, and good chemical stability, but they also have large surface areas (280˜420 m2/g) and high surface activities. Ultra dispersed diamonds have been proposed to be used as a lubricating additive. It is often desirable to improve the dispersion of the ultra dispersed diamonds in solvents in order to increase their applicability. However, ultra dispersed diamonds easily aggregate to micro size, and lose their unique features as nano-particles due to the high specific surface energy. The aggregated ultra dispersed diamonds have size of more than several micrometers, thereby exhibiting inferior mobility, friction, and dispersibility properties.
In China Patent Application No. 02115230.6, nano-diamonds were modified by a specific silane reagent. Although the method improved the stability of the nano-diamonds in medium, the cost of the silane reagent is high, and the reaction time is long, thus, limiting industrial applications. In another example such as China Patent Application No. 02139764.3, surfactant was added into nano-diamonds by gas flow pulverization, high pressure liquid flow pulverization, or bead milling. By physical pulverization or mechanical milling, the nano-diamonds were equally dispersed into a solution. However, because the surfactant is absorbed on the surface of the nano-diamonds, the nano-diamonds can only be dispersed into some specific solvents, and therefore, the applications thereof are limited.
U.S. Pat. Pub. No. 2008248979A1 discloses a lubricant composition, including diamond nano-particles and dispersants, can reduce friction coefficient. The diamond nano-particles can be dispersed in lubricating oil by physisorption effect in the presence of a dispersant. However, a dispersant added in an excess amount would detrimentally affect the friction properties thereof, and the physisorption effect is apt to be unstable under a high temperature. Therefore, after prolonged operation, the ultra dispersed diamonds re-aggregate to a agglomeration, having a micro size, due to desorption of dispersant under a high temperature. The aggregation not only reduces friction properties but also causes machines to be scratched.
Accordingly, it is highly desirable to develop an effective technique to stably disperse the ultra dispersed diamond into base oil in the absence of a dispersant (or surfactant).